Alternate Flow Heat Recovery (AFHR) is a decentralised mechanical ventilation system designed to recover heat from exhaust air and transfer it to incoming fresh air. Unlike traditional balanced mechanical ventilation with heat recovery (MVHR) systems that use two separate ducts for supply and extract, AFHR units operate in cycles, typically using a single fan and a ceramic heat exchanger.

Explanation: In the UK house building, residential retrofit, home renovation, and extension sectors, AFHR offers a more straightforward and often more cost-effective alternative to full MVHR systems, particularly for individual rooms or smaller properties where extensive ductwork is impractical.

Here's how it works:

1. Extract Cycle (Approx. 70 seconds): The fan extracts warm, stale air from the room and passes it through a ceramic heat exchanger. As the air passes, the heat exchanger absorbs the thermal energy.

2. Supply Cycle (Approx. 70 seconds): The fan reverses direction, drawing fresh, cooler air from outside. This incoming air passes through the now warm ceramic heat exchanger, absorbing the stored heat before being supplied into the room.

This cyclical operation ensures continuous ventilation while significantly reducing heat loss. AFHR units are often installed in pairs on opposite walls to create a more balanced airflow across a space, or individually in smaller rooms.

Practical Examples:

• Residential Retrofit: In a Victorian terraced house undergoing retrofit, installing AFHR units in bedrooms and living areas can provide effective ventilation and heat recovery without the need for extensive structural changes to accommodate ductwork. This is particularly beneficial in properties with solid walls where installing traditional MVHR ducts would be disruptive and costly.

• Home Extension: When adding a new kitchen-diner extension to a semi-detached house, an AFHR unit can be incorporated into the design to ensure adequate fresh air supply and heat recovery, helping to meet the ventilation requirements outlined in Approved Document F (Ventilation) of the Building Regulations.

• Single Room Renovation: For a loft conversion creating a new bedroom and en-suite, a single AFHR unit in the bedroom can provide targeted ventilation and help manage condensation, contributing to a healthier indoor environment.

AFHR systems are generally quieter than extract-only fans and provide better indoor air quality by continuously supplying filtered, tempered fresh air. They contribute to meeting the requirements of Approved Document L (Conservation of Fuel and Power) by reducing the energy demand for heating, as less heat is lost through ventilation.